In weaving textiles, the threads put on the loom in the linear direction are known as the warp. Because of the rubbing and abrasion during the weaving operation, the thread used for warp must be sized to tie down loose ends which might become entangled, to provide resiliency, to provide some added strength, and to lubricate the warp. Size is typically applied during the warping operation by the slasher, a multi-function apparatus that coats the warp threads, squeezes them through rollers to remove excess size, and dries them before they go into the loom. Since many warp sizes, such as starch, congeal at room temperature, usually the size is applied hot. Size is applied to a wide variety of fibers, both natural and man-made. Warp size is used on cotton, polyester, nylon, rayon, various spun yarns and continuous filament threads. The amount of size used would depend on the make-up of the size and on the fibers being treated. Thus, a natural spun yarn such as cotton requires large amounts of natural size such as starch (up to 15% and more add-on), whereas continuous filament thread, where the size acts primarily as a lubricant, is treated with a synthetic size such as polyvinyl alcohol at only a 11/2 to 3% add-on level. "Add-on" is the amount of size (% dry basis) put on a fabric based on the weight of the fabric.
One property of a size which is critical is its ability to be easily removed; i.e., after weaving the fabric must be washed to remove the size so that it will not interfere with any subsequent fabric treatment such as printing. Starch is typically removed by enzyme treatment; polyvinyl alcohol (PVOH) by hot scours.